The present invention relates to the regeneration of spent fixer solution resulting from the processing of black-and-white films.
The photographic processing of various types of films, including lithographic and x-ray films involves the development of an image by reduction of silver halide crystals to metallic silver and the removal of the unused silver halides from the film. Various photographic fixer solutions are used in the industry to dissolve the residual silver halide. The most common form of photographic fixer solution is one which contains a thiosulphate such as sodium thiosulphate or ammonium thiosulphate as active ingredient. A spent fixer solution is one which has been used for fixing film and in this specification and claims this is the meaning which will be given to the term "spent fixer solution".
Silver recovery from spent fixer solution is customarily done by electrolysis. Such processes suffer from a number of disadvantages. Apart from the relatively high capital and maintenance costs, and the relatively low rate of throughput, the process is also relatively inefficient. As full recovery is being approached, side reactions become increasingly important, leading to the precipitation of sulphur. Usually it is possible to recover only of the order of 97 to 99 percent of the silver originally present.
Although it is possible to recover the residual silver, e.g. with steel wool, such silver is considered by many not to be worthwhile. The barren fixer, with or without residual silver (which is highly toxic to lower forms of life), is usually disposed of into the sewer, annually adding tonnage quantities of barren fixer solution, an environmentally unfriendly material, to the elecrolyte load of urban wastewater.
In general, spent fixer solution which has been subjected to electrolysis is unsuitable for re-use. For not only is part of the thiosulphate ion oxidised at the anode while the silver is being deposited at the cathode, but other sulphur species are also formed, and their presence in the electrolysed fixer solution tends to lead to the deposition of harmful deposits in the processing equipment. Electrolysis produces a relatively dark color in the fixer solution (probably by electrolytic oxidation of some of the additives), and both ammonium thiosulphate and some of the additives require replenishment if the use of the fixer is to be extended.
Another small-scale method for silver recovery comprises passing the spent fixer solution through a column packed with a cartridge of an alloy fabricated into a fibrous form resembling steel wool. Recovery is however usually incomplete and the treated fixer solution is unsuitable for re-use. Moreover, the method adds heavy metals to the effluent, which customarily goes straight down the drain.
South African Patent No. 90/7735 teaches precipitation of the silver as silver sulphide with a solution of the sulphide or hydrosulphide of an alkali metal or ammonium. The mother liquor drawn off after the silver sulphide has settled out is claimed to be regenerated and suitable for re-use. But the method suffers from a number of disadvantages. Addition of the sulphide or hydrosulphide has to be carefully controlled, for excess sulphide leads to precipitation of sulphur. The spent fixer solution cannot be regenerated at source (e.g. in large hospitals), in the first instance because there is always the accompanying risk of foul-smelling, highly toxic hydrogen sulphide gas being released during processing, particularly if the pH of the fleer solution is at the lower end of the typical operating range. The reagent itself is characterised by a smell of hydrogen sulphide.
Also, the recovered silver sulphide has to be roasted before the silver may be smelted out, in the process sending the sulphide into the atmosphere as sulphur dioxide (the "acid rain" gas), unless steps are taken to scrub it out from the gases given off.